A lion broke the neck of an intern at a US animal park after opening its cage with its paw and attacking the woman as she cleaned its holding area, authorities believe.

It remains unclear how the lion and 24-year-old Dianna Hanson were in the same place at the same time, Fresno County coroner David Hadden said on Thursday.

"The lion had been fed, the young woman was cleaning the large enclosure, and the lion was in the small cage. The gate of the cage was partially open, which allowed the lion called Cous Cous to lift it up with his paw," Hadden said, based on a briefing from investigators. "He ran at the young lady."

The lion broke Hanson's neck, probably with a swipe of a paw, killing her instantly, Hadden said.

Hanson was talking with a colleague on a mobile phone just before she was killed on Wednesday, the coroner said. The colleague became concerned when the conversation ended abruptly and Hanson failed to call back. The colleague then called authorities after checking on Hanson.

The intern sustained numerous bites and scratches, all inflicted after she died, the postmortem examination revealed.

"Which means the young lady … wasn't alive when the lion was tossing the body about," said Hadden.

Hanson had been working for two months as an intern at Cat Haven, a private zoo in California. Her father, Paul Hanson, described her as a "fearless" lover of big cats. She died doing what she loves, he said.

Hanson told her father she was frustrated that Cat Haven did not allow direct contact with animals.

"She was disappointed because she said they wouldn't let her into the cages with the lion and tiger there," Paul Hanson said.

The owner of the zoo said on Thursday that safety protocols were in place, but he would not discuss them because they are a part of the police investigation. Dale Anderson said that he is the only person allowed in the enclosure when lions are present.

"We have been incident-free since 1998 when we opened," Anderson said.

Cat Haven breeds and keeps lions, tigers, jaguars, lynx and other exotic cats and takes them out for public appearances. A recent television report showed a reporter petting one of the animals.

The US agriculture department, which enforces the federal Animal Welfare Act, said: "We're looking at whether the animal was acting in a manner leading up to that situation that maybe the staff should have been aware of. Was it being fed properly? Was it under undue stress?"

Department inspectors conduct multiple unannounced inspections of Cat Haven each and every year and had never had found a violation, a spokesman said.